System for measuring light transmittance through absorptive or diffusive media

ABSTRACT

A system and process are disclosed for measuring the transmittance or opacity of a smoke plume discharged from a smokestack or diesel engine exhaust. Measurement is accomplished by passing light pulses through the smoke plume and detecting the remaining energy with a photoelectric detector. The effect of scattered light is eliminated. An electrical signal proportional to the unabsorbed light received at the detector is displayed on a calibrated opacity meter.

United States Patent n113,632,209

[72] Inventor Edward F. Kingman [56] References Cited [21] A I N 229callf- UNITED STATES PATENTS PP Filed p 1970 2,198,971 4/1940 Neufeld356/208 P t nted Jan. 4, 1972 Primary Examiner-Ronald L. Wibert [73]Assignee The Susquehanna Corporation s a t Examiner-Evans F. L.

Fairfax County, Va. AttorneyMartha 1... Ross [54] SYSTEM FOR MEASURINGLIGHT TRANSMITTANCE THROUGH ABSORPTIVE 0R ABSTRACT: A system and processare disclosed for measur- DIFFUSXVE MEDIA v 11 Cl mg the transmittanceor opacity of a smoke plume discharged arms, 1 Drawing Fig.

from a Smokestack or diesel englne exhaust. Measurement 15 [52] US. Cl356/201, accomplished by passing light pulses through the smoke plume250/2l8, 356/205, 356/207 and detecting the remaining energy with aphotoelectric del lllt- Cl t ln 21/26 tector. The efiect of scatteredlight is eliminated. An electrical Field of Search 250/218; signalproportional to the unabsorbed light received at the de- 2 tector isdisplayed on a calibrated opacity meter. 7

3a 34 /6 M ELM Y 28 I ma $044: 26 m I 4 24 3a T i 22U6/-/7 SOURCE T/Jz20-7ARG7 l y f ,/6 46 l zeta Aw.

mu saw: 40.1.

' PATENIEI] JAN 4 1912 I w r INVENTOR Emma; E /fl/VGM/M/ I "ATTORNEYSYSTEM FOR MEASURING LIGHT TRANSMITTANCE THROUGH ABSORPTIVE OR DIFFUSIVEMEDIA BACKGROUND OF THE INVENTION The present invention relates broadlyto the field of densitometers or transmissometers and more particularlyto a system and process for measuring the amount of light energyreceived through a medium being measured from a source of known orconstant intensity. A particular use is in the measurement of thetransmittance or opacity of smoke plumes.

Around the turn of the century a procedure for evaluating black smokewas devised by a French engineer, Maximilian Ringelmann, and is stillwidely used. The luminance of the smoke plume is compared to theluminance of four white charts on which are black grids obscuring 20,40, 60 and 80 percent of the charts surfaces.

A comparison is made between (l) the amount of light transmitted to theobserver through the black smoke from the portion of sky on its far sideand (2) the amount of light from a different and wider area of sky andfrom the sun, in whatever position it happens to be, reflected to theobserver from the white areas of the chart. Even if the smoke does notscatter an appreciable amount of sun and sky light toward the observer,the limitations of such a comparison between totally differentquantities has long been recognized. It is readily seen that there issome question as to the accuracy with which an observer can use suchthings as Ringelmann charts having various shades of blackness todetermine the opacity of nonblack plumes. Nevertheless, the Ringelmanncharts remain the basis for smoke legislation and control in most, ifnot all, industrial nations. Other methods of comparison, such as thoseusing filters as well as direct transmission measurement methods havebeen tried but have been found to be erroneous if the smoke is not blackor if it scatters appreciable light to the observer or to theinstrument.

As regards a smoke plume which is nonblack or has some color, thereappears to be no recognized method of truly evaluating it. In whitesmoke, for example, the plume is often brighter than light from the skybackground because of the scattering of light the plume receives fromthe rest of the sky and from the sun. A study of the phenomenon of blackand nonblack plumes was recently completed by the US. Public HealthService in cooperation with the Edison Electric Institute. A report ofthese studies is presented in Optical Properties and Visual Effects ofSmokestack Plumes," No. 999-AP-30, published in 1967 by the US.Department of Health, Education and Welfare. Because of the growinginterest in air pollution and because of the inadequacies of prior arttechniques and instruments, there is a compelling need for an objectiveinstrumental method and system for measuring plume emissions in thefield.

SUMMARY OF THE INVENTION This patent application discloses a novelsystem and method for measuring the opacity of a medium even in thepresence of scattered light and regardless of the color or lack of colorof the medium. Using a smoke plume as an example, the difference in theluminance between a pair of contrasting targets viewed through the plumeand viewed clear of the plume is measured. The targets consist of anoptically flat black surface and a light source positioned in the centerof the black target. The source is flashed at a constant rate, and adetector positioned opposite the target alternately sees the lightsource and the contrasting black target. A calibrated measuring circuitand meter process the energy received at the detector to provide anindication of the light attenuated or absorbed by the medium beingmeasured. The calibration and construction of the system eliminates theeffect of scattered light in the medium and provides a true measurementof transmittance. Thus, as in the particular instance of the opacitymeasurement of diesel exhaust, it now becomes a simple matter toascertain whether or not the exhaust smoke falls within or withoutstandards established by state or federal regulations.

BRIEF DESCRIPTION OF THE DRAWING The foregoing and other objects,features and advantages of the invention will be apparent from thefollowing more particular description of a preferred embodiment of theinvention, as illustrated in the sole accompanying drawing showing thelight transmitting and receiving circuits forming the system of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The instrument shown in thedrawing consists of two basic units, one being the transmitter 10,including a light source and contrasting target, and a receiver 12,comprising a detector and a processing and indicating circuit. Thesystem is of rugged construction, lightweight, and readily portable formaking field measurements, such as for the measurement of opacity ofdiesel smoke plumes, which example will be used in the presentdescription. A self-contained battery supply 13 is connected into thecircuit by ganged switch 14 in the positive and negative supply lines 16and 18, respectively. The battery 13 and power switches 16 and 18 can becontained in either the transmitter or receiver section, or separatetherefrom, if desired.

The transmitter 10 has a target 20 shown here as being circular indesign and large enough to fill the view of the detector of receiver 12.The target 20 is preferably painted with an optically flat-black paintto provide a black or dark target reference. Centered in the target 20is a light source 22 which functions as a second target. Source 22 ispreferably a lightemitting diode having a substantially constantintensity of radiation. This solid state light source 22 is pulsed by aconventional unijunction oscillator 24, and with target 20 providesalternate, contrasting targets. Resistors 26; 28 and 30 and capacitor 32form the circuit elements of the unijunction oscillator, which isdesigned to run at a preferred rate of Hz. Resistor 34 and capacitor 36act as a filter for the power supply. Switch 38, normally closed, isopened when it is desired to make a full-scale calibration of thesystem.

In the receiver 12, the detector 40 is mounted in a recessed fashion inplate 42. This detector is aimed at the light-emitting diode 22 so thatthis diode and the target 20 fill its view. Detector 40 is preferably aphotosensitive diode, such as a silicon photodiode, whose current isprecisely controlled by the in tensity of received light. Detector 40and the targets 20 and 22 are spaced apart to permit the passage of asmoke plume therebetween.

The cathode of diode 40 is connected to positive supply line 16, and itsanode is connected to negative supply through a large-valued resistor44. The anode of photosensitive diode 40 is also connected to inputcapacitor 46 through which the signals from diode 40 are applied to theamplifying and indicating circuits. Capacitor 46 filters or blocks outsteadystate components, thus permitting only the passage of the impulsesreceived from diode 40. The output of capacitor 46 is connected to inputresistor 48 constructed as a potentiometer. This potentiometer controlsthe input level to the amplifying circuits and provides a means formaking a zero calibration of the meter circuit, as later described. Thetap of potentiometer 48 is connected to the noninverting input ofamplifier 50 through resistor 52.

Amplifier 50 is an operational amplifier and provides a high, stablegain to input signals. The remaining inputs to the amplifier 50 areconventional bias leads and feedback loops for such purposes as fastrecovery and gain control. The output of amplifier 50 is connectedthrough resistor 54 and diode 56 to capacitor 58. Connected in parallelwith capacitor 58 is a resistor 60 of high resistance, preferably on theorder of l megohm. Resistor 60 provides a leakage or bleed path forcapacitor 58. The voltage level at capacitor 58 is applied to thenoninverting input of another operational amplifier 62 which is designedto provide essentially unity gain and a high input impedance to thisinput voltage.

The output of amplifier 62 is applied through resistor 64 and diode 66to capacitor 68. Connected at the junction of diode 66 and capacitor 68is a resistor 70 and a normally opened switch 72 used for zerocalibration and to reset the circuit, as later described.

The voltage level at capacitor 68 is applied to the gate input of afield-effect transistor 74. This transistor has a resistor 76 connectedbetween its source electrode and negative battery. Transistor 74 isconnected as a source follower, and the voltage on capacitor 68 appearsat its source and, accordingly, across resistor 76. A feedback pathincluding resistor 78 is applied from this source resistor 76 to theinverting input of operational amplifier 62 to provide a high impedanceat the inverting input and thereby cause the high input impedance at thenoninverting input.

Meter 80 is connected in a bridge circuit including transistor 74 andresistor 76 on one side of the bridge and potentiometer 82 on the otherside of the bridge. Resistors 84 and 86 are conventionalcurrent-limiting and shunt resistors, respectively.

The operation of the system can be best understood from a preliminarydiscussion of the practical measurement of opacity. The basis for anopacity meter can be established even in the presence of scattered lightby measuring and comparing the luminance difference between a pair ofcontrasting targets viewed clear of the plume and through the plume. Thevalue of opacity is one minus the value of transmittance, the latterbeing a property of transmitted light that can be used to differentiateand characterize plumes. With regard to the luminance of targets,transmittance is defined as:

where B is the luminance of a source, 13 is the luminance of itsbackground, both B and 8 viewed clear of the plume; B, is the apparentluminance of a source, B is the apparent luminance of its background,both viewed through a plume. In this transmittance formula, both B, andB have the same added increment of light caused by scattering, and thescattered light cancels out.

The present system is designed to be calibrated to make the contrast ordifference in the luminance clear of the plume, i.e., B,B equal to 100.The contrast between the apparent luminances, as viewed through thesmoke plume, is then:

for the medium being measured.

In the actual design of the system, the meter face has been designed toread opacity directly, form to 100 percent, instead of transmittance. Ifdesired, the latter can be also read off of the meter or the meter faceprovided with another scale because 0 and opacity equals 100 percenttransmittance and 100 percent opacity equals 0 transmittance. Capacitor46, previously discussed, blocks all steady-state input signals and thusreferences the contrast between apparent luminances to 0. The effect ofscattering is eliminated.

Prior to the use of the system in measuring the opacity of a smokeplume, it is calibrated to make the contrast clear of the plume equal to100. One hundred percent opacity is calibrated first. Gang switch 14 isclosed, applying power to the system. Switch 38 is opened, turning offoscillator 24 to prevent the transmission of light pulses from diode 22.With no light pulses arriving at diode 40 in the receiver, at best onlya steady-state signal will appear at the input to capacitor 46.

However, this capacitor blocks this signal; and a 0 voltage appears atpotentiometer 48 and, therefore, at the input to amplifier 50. Capacitor58 is discharged as is capacitor 68 at the output of amplifier 62. Withno voltage applied to resistor 76 from the detecting and amplifyingcircuitry, potentiometer 82 is adjusted to give a full scale reading of100 percent opacity.

For 0 calibration, switch 38 is closed. Oscillator 24 begins to functionand applies energizing pulses to light-emitting diode 22 at the rate of100 Hz. Switch 72 is also closed and capacitor 68 begins to charge. Atthe same time the light pulses from diode 22 are detected at diode 40 inthe receiver, and amplified current pulses out of amplifier 50 begin topositively charge capacitor 58. The voltage level attained at thiscapacitor will be proportional to the voltage at the tap ofpotentiometer 48. Amplifier 62 applies unity gain to the positivevoltage at capacitor 58, and this positive voltage is reflected at itsoutput. Diode 66 will, therefore, limit the level to which capacitor 68can charge, i.e., the level on capacitor 68 cannot exceed the level oncapacitor 58, or else diode 66 will conduct to bring the charge oncapacitor 68 down to the level of capacitor 58. The voltage on capacitor68 now appears at resistor 76, and the meter will be at or close to azero reading. If any adjustment of the meter is needed to give an exactzero reading, the tap of potentiometer 48 can be moved. The effect is tochange the voltage on capacitor 58 and, accordingly, at capacitor 68 andresistor 76. It can be seen from the above description that the degreeof contrast between the signal and no-signal conditions, or light anddark targets, is not critical because of the systems ability tocalibrate for l0() and 0 percent readings.

With the system now calibrated for target contrast clear of any exhaustplume, the target 20 and plate 42 can now be moved by any convenientmeans so that the exhaust plume from a diesel engine will span or passthrough the space between these two members. The system is first resetby closing switch 72, and a full charge is applied to capacitor 68. Ifthe smoke being tested is transparent and free of light attenuating orabsorbing particles, then the circuit will function much the same as wasdescribed in the 0 calibration mode so that sufficient charge remains oncapacitor 68 to cause a 0 percent opacity reading at meter 80. Assume,however, the usual case where the smoke is black, nonblack or colored.The light pulses out of diode 22 will be attenuated or absorbed by thesmoke causing a reduction in the intensity of the pulses received bydiode 40. Because the current through this latter diode is controlled bythe intensity of the received light, the amplitude of these pulses willbe less than realized for a zero reading. These pulses are appliedthrough capacitor 46 and across potentiometer 48. Amplifier 50 amplifiesthese pulses and applies their peak positive level throughforward-biased diode 56 into storage capacitor 58. Thus, the voltagestored on capacitor 58 is proportional to the voltage tapped atpotentiometer 48.

With the pulses being applied at a Hz. rate, the charge on capacitor 58accumulates quickly to the full value of these amplitude peaks. Thislevel appears at the output amplifier 62. The cathode of diode 66 willbe at a lower potential than its anode causing this diode to becomeforward-biased. Capacitor 68 now discharges through diode 66 intoamplifier 62 until the voltage level at this capacitor balances theoutput of the amplifier. At such time, diode 66 becomes reverse-biasedand cuts off.

The voltage on capacitor 68 appears at resistor 76 in the meter circuitand unbalances the bridge circuit of the meter 80 causing current flow.The needle of meter 80 will now move away from the 0 percent point andcome to rest at a reading of opacity indicative of the opacity of thesmoke being tested.

The RC circuit combination of capacitor 58 and resistor 60 provides atime constant of about 0.1 second and thereby permits only a very slightdischarge of capacitor 58 between each pulse being applied at the 100Hz. rate. Nevertheless, if opacity should increase while the measurementis being taken, capacitor 58 will discharge quickly down to this newlevel, thereby causing further discharge of capacitor 68 through diode66 to give a higher opacity reading at meter 80. If, on the other hand,opacity should decrease during the reading, or if there is any doubt asto whether a completed reading is an accurate one, it is a simple matterto close switch 72 to recharge capacitor 68 fully and thereby return themeter to a 0 percent reading in preparation for a new measurement.

The target 20 in the transmitter is preferably painted or coated with anoptically flat black covering to give a good contrast between lightsource 22 and this target. The degree of contrast is not critical,however. ln fast, the targets themselves may become appreciably dirty orworn, or smoke particles may coat the surfaces of diodes 22 and 40reducing contrast or causing a reduction in light intensity withoutaffecting system operation, so long as the system can be calibrated for0 and full scale meter deflections. Also, since the measurement ofopacity does depend on difference readings, the need for critical orprecision components is obviated, except perhaps for diodes 22 and 40,since all readings will be affected in the same manner.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit or the scope of theinvention.

What is claimed is:

1. A system for measuring the opacity or transmittance of a mediumcomprising,

a light transmitter, including,

a. a light source for emitting light,

b. a target for providing an optical contrast to said light source,

c. and means for causing said light to be emitted from said lighttransmitter as pulses of light;

a receiver, including,

a. a photosensitive detector spaced from said light source and targetand positioned to have its field of view filled thereby, for providingan output in proportion to the intensity of light received through saidmedium,

b. a processing circuit connected to said detector for providing anelectrical signal proportional to the detector output, said processingcircuit including,

i. means for substantially eliminating the effect of scattered lightcaused by said medium and received at said detector,

c. and means connected to said processing circuit for providing asensible indication of opacity or transmittance of the medium;

said light source and said detector defining the sole light path betweensaid transmitter and receiver through said medium for receipt of lightby said detector.

2. A system as claimed in claim 1 wherein,

a. said causing means comprises means for intermittently energizing saidlight source to cause said light source to emit pulses of light.

3. A system as claimed in claim 2 wherein,

a. said medium is a smoke plume, and

b. said providing means is a meter for providing a direct reading inpercent opacity of said smoke plume.

4. A method for measuring the opacity or transmittance of a medium bythe use of a light source and an optically contrasting target and adetector spaced therefrom, and a measuring circuit connected to thedetector, comprising:

a. calibrating said circuit so that the difference in luminance betweensaid light source and contrasting target when viewed by the detectorclear of the medium is a fixed value,

b. pulsing said light source to provide alternately to the detector asource of light and an optically contrasting target as viewed throughthe medium,

c. causing said medium to flow in a path between said light source andcontrasting target and said detector,

d. detecting the apparent luminances between said light source andcontrasting target when viewed by said detector through said medium,

e. electrically processing the detected apparent luminances to rovide adifference signal, and f. re erencing said difference signal to saidfixed value to give an indication of the opacity or transmittance ofsaid medium.

5. A method as claimed in claim 4 further comprising the step of:

a. Eliminating the effect of scattered light through the medium whileelectrically processing said detected apparent luminances.

6. A method as claimed in claim 5 where said medium is a smoke plume andfurther comprising the step of:

a. Providing for said indication a direct reading in percent opacity ofsaid smoke plume.

7. A system for measuring the opacity or transmittance of a mediumcomprising,

a light transmitter, including,

a. a light source for emitting light,

b. a target for providing an optical contrast to said light source,

c. means for causing said light to be emitted from said lighttransmitter as pulses of light;

a receiver, including,

a. a photosensitive detector spaced from said light source and targetand positioned to have its field of view filled thereby, for providingan output in proportion to the intensity of light received through saidmedium,

b. a processing circuit connected to said detector for providing anelectrical signal proportional to the detector output, said processingcircuit including,

i. means for calibrating said receiver with reference to said lightsource and contrasting target clear of the effect of said medium,

c. and means connected to said processing circuit for providing asensible indication of opacity or transmittance of the medium.

8. A system as claimed in claim 7 wherein,

a. said causing means comprises means for intermittently energizing saidlight source to cause said light source to emit pulses of light.

9. A system as claimed in claim 8 wherein,

a. said processing circuit includes means for substantially eliminatingthe effect of steady-state components received at said detector, and

b. said light source and said detector define the sole light pathbetween said transmitter and receiver through said medium for receipt oflight by said detector.

10. A system as claimed in claim 9 wherein,

a. said target is black, and

b. said light source is approximately centered in said black target withreference to the field of view of said detector.

11. A system as claimed in claim 10, wherein a. said medium is a smokeplume, and

b. said providing means is a meter for providing a direct reading inpercent opacity of said smoke plume.

1. A system for measuring the opacity or transmittance of a mediumcomprising, a light transmitter, including, a. a light source foremitting light, b. a target for providing an optical contrast to saidlight source, c. and means for causing said light to be emitted fromsaid light transmitter as pulses of light; a receiver, including, a. aphotosensitive detector spaced from said light source and target andpositioned to have its field of view filled thereby, for providing anoutput in proportion to the intensity of light received through saidmedium, b. a processing circuit connected to said detector for providingan electrical signal proportional to the detector output, saidprocessing circuit including, i. means for substantially eliminating theeffect of scattered light caused by said medium and Received at saiddetector, c. and means connected to said processing circuit forproviding a sensible indication of opacity or transmittance of themedium; said light source and said detector defining the sole light pathbetween said transmitter and receiver through said medium for receipt oflight by said detector.
 2. A system as claimed in claim 1 wherein, a.said causing means comprises means for intermittently energizing saidlight source to cause said light source to emit pulses of light.
 3. Asystem as claimed in claim 2 wherein, a. said medium is a smoke plume,and b. said providing means is a meter for providing a direct reading inpercent opacity of said smoke plume.
 4. A method for measuring theopacity or transmittance of a medium by the use of a light source and anoptically contrasting target and a detector spaced therefrom, and ameasuring circuit connected to the detector, comprising: a. calibratingsaid circuit so that the difference in luminance between said lightsource and contrasting target when viewed by the detector clear of themedium is a fixed value, b. pulsing said light source to providealternately to the detector a source of light and an opticallycontrasting target as viewed through the medium, c. causing said mediumto flow in a path between said light source and contrasting target andsaid detector, d. detecting the apparent luminances between said lightsource and contrasting target when viewed by said detector through saidmedium, e. electrically processing the detected apparent luminances toprovide a difference signal, and f. referencing said difference signalto said fixed value to give an indication of the opacity ortransmittance of said medium.
 5. A method as claimed in claim 4 furthercomprising the step of: a. Eliminating the effect of scattered lightthrough the medium while electrically processing said detected apparentluminances.
 6. A method as claimed in claim 5 where said medium is asmoke plume and further comprising the step of: a. Providing for saidindication a direct reading in percent opacity of said smoke plume.
 7. Asystem for measuring the opacity or transmittance of a mediumcomprising, a light transmitter, including, a. a light source foremitting light, b. a target for providing an optical contrast to saidlight source, c. means for causing said light to be emitted from saidlight transmitter as pulses of light; a receiver, including, a. aphotosensitive detector spaced from said light source and target andpositioned to have its field of view filled thereby, for providing anoutput in proportion to the intensity of light received through saidmedium, b. a processing circuit connected to said detector for providingan electrical signal proportional to the detector output, saidprocessing circuit including, i. means for calibrating said receiverwith reference to said light source and contrasting target clear of theeffect of said medium, c. and means connected to said processing circuitfor providing a sensible indication of opacity or transmittance of themedium.
 8. A system as claimed in claim 7 wherein, a. said causing meanscomprises means for intermittently energizing said light source to causesaid light source to emit pulses of light.
 9. A system as claimed inclaim 8 wherein, a. said processing circuit includes means forsubstantially eliminating the effect of steady-state components receivedat said detector, and b. said light source and said detector define thesole light path between said transmitter and receiver through saidmedium for receipt of light by said detector.
 10. A system as claimed inclaim 9 wherein, a. said target is black, and b. said light source isapproximately centered in said black target with reference to the fieldof view of said detector.
 11. A system as claimed in claim 10, whereina. said medium is a sMoke plume, and b. said providing means is a meterfor providing a direct reading in percent opacity of said smoke plume.